Motion of the human body is generally controlled by the contraction of various muscles. Muscular contraction is caused when electrical signals (i.e., “action potentials”) travel from the brain, through the central and peripheral nervous systems, and into the target muscle tissue to effect contraction of structural units within the muscle tissue—known as sarcomeres. For example, motion of the fingers of the human hand is controlled by several muscles in the forearm. These forearm muscles contract when electrical signals are sent from the brain, through the central nervous system (i.e., the spine), through the peripheral nervous system (i.e., the arm), and into the muscle tissue to trigger the contraction of the sarcomeres within the muscle tissue. The forearm muscles are connected to the bones of the fingers via tendons such that when the forearm muscles contract, the fingers bend.
To generate an “action potential,” a gradient of ions creates a voltage difference across an axon of a neuron. When a threshold voltage difference is exceeded due to the ion gradient, an electrical wave propagates down the length of the axon until it reaches the end of the axon, which may be within muscle tissue. The neuron releases acetylcholine and transfers the action potential to the muscle tissue. The electrical signal will travel through the tissue and trigger the contraction of the individual sarcomeres. One synapse generally controls an entire muscle fiber. One motor neuron usually controls several adjacent muscle fibers. A group of fibers under the control of a single motor neuron is known as a motor unit.
Prosthetic devices have long been used to replace missing body parts, such as hand, arms, and legs. However, these prosthetic devices may be immobile and thus lacking in the same utility that a natural body part imparts. Other prosthetic devices known in the art may use various sensors (e.g., electrodes) to detect the action potential propagation through a muscle—also called electromyography—and use the detection of the action potential as an input signal to control motors that allow the prosthetic device to move. In an example, a prosthetic hand may include one or more motors that control the flexion of individual fingers to mimic the natural flexion of the fingers of a biological hand.
One example of a prosthetic hand known in the art is the Dextrus hand of the Open Hand Project. The Dextrus hand is a prosthetic hand that offers some of the functionality of a human hand by using electric motors instead of muscles and steel cables instead of tendons all packaged within a polymer housing. The electric motors are controlled by electronics (such as a microprocessor) that utilize stick-on electrodes to read signals from the muscles in the forearm, which can control the fingers of the prosthetic hand, causing it to open or close. However, previous implementations of the Dextrus hand were only capable of gripping objects and did not have the capability of fine motor control of the fingers to generate a stroking motion. Thus, a user of previous implementations of the Dextrus hand would have to grip an object and use body parts outside of the prosthetic (e.g., shoulder and/or elbow) to create motion of the object being held. This is inconvenient for the user when performing tasks that would more suitably be performed using fine motor control of the fingers.
Other known prosthetic hands include the i-limb, which is an externally powered prosthesis often controlled by myoelectric signals. However, the i-limb only has preprogrammed grips and does not have the capability of fine motor control of the fingers to generate a stroking motion. A user would have to grip an object with a preprogrammed grip and use joints and/or muscles outside of the finger joints/muscles (such as the shoulder and/or elbow) to create motion of the object being held. Another prosthetic hand, the bebionic3, similarly has preprogrammed grips that are used to hold an object, but does not have the capability of fine motor control of the fingers to generate a stroking motion. Again, a user would have to grip an object with a preprogrammed grip and use joints and/or muscles outside of the finger joints/muscles (such as the shoulder and/or elbow) to create motion of the object being held.
While advances in prosthetic technology have allowed these devices to be manufactured efficiently at low cost, the motion of these prosthetic devices can be difficult to control. Among other issues with control of the prosthetics, no prosthetics have yet addressed the issue of fine motor control of the fingers to generate a stroking motion. Such a motion may be useful for everyday tasks such as writing, painting, eating, brushing teeth, etc. to allow a user to have a more normal life post-amputation of an appendage. Thus, a need exists for systems and methods of controlling prosthetic devices in a way that mimics the fine motor control of the body part the prosthetic intends to replace. With respect to a prosthetic hand, for example, a need exists for systems and methods for fine motor control of the fingers to allow for delicate motion of the fingers, such as forming strokes with a writing utensil.